The known art for the selective extraction of desired materials from feed streams falls into three general categories. First among these is resin bead extraction typified by ion exchange resins which have been used selectively to extract species such as transition metal ions from aqueous solutions. Specific chelating resins with selectivity for one or more desirable metal ions are also known but are difficult to manufacture, are consequently expensive, and suffer from severe bead attrition due to physical breaking up of the beads in flow systems. The difficulty of attaching desirable chelating groups to available macroporous resin beads limits the quantity and selectivity of the materials which can be made using this technique.
One relatively recent publication "Copper-Selective Ion-Exchange Resin with Improved Iron Rejection" R. R. Grinstead, Journal of Metals, Vol. 31, No. 3 (1979) 13-16, describes a then improved chelating resin for copper/iron ion solutions, highly selective for copper.
A second process involves solvent extraction which is typically used to recover mineral values employing selective extractants soluble in solvents, immiscible with the feed solvent. Large volumes of solvent and extractant inventory are required, however, and valuable extractant is commonly lost due to solubility in the feed stream and entrainment losses resulting from microfine solvent/extractant particles which do not coalesce in the solvent/feed separators.
More recently, patent and open literature references have suggested the use of membranes as separators to avoid the solvent/extractant losses due to entrainment.
U.S. Pat. No. 3,808,267 teaches a membrane process for recovery of C.sub.2 to C.sub.4 carboxylic acids from dilute aqueous solution with one side of a microporous membrane while the other side is contacted with a liquid organic solvent for the acid.
"Membrane-Based Solvent Extraction for Selective Removal and Recovery of Metals", B. M. Kim, Journal of Membrane Science, 21 (1984) 5-19, describes a process for stripping industrial metals from wastewater. The process employs two modules, the first being for extraction through which the aqueous waste water stream flows, routed through thousands of very thin hollow fibers. An organic solvent containing a liquid ion-exchange material flows between the fibers and collects metal ions migrating through the pores in the fibers. The organic solvent stream then flows through a stripping module wherein an acid, base or salt solution, as appropriate, removes the metal ions.
In both cases these references discuss the use of immiscible solvents as the means of separating the extractant from the feed stream since the extractants used are small enough to permeate through the membranes employed. Both solvent and extractant soluble losses do occur in these systems and result in organic pollution of the feed stream.
Membrane methods comprise the third category for the selective removal of materials from solutions. In most instances these methods have been pressure driven ultrafiltration or reverse osmosis processes as opposed to the affinity dialysis process which is concentration gradient driven. U.S. Pat. No. 4,163,714 describes the preparation of pressure driven affinity adsorption membranes from a membrane filter which is composed of an insoluble matrix polymer or interpolymer complex. The membranes are employed by passing material-containing solutions therethrough where one or more of the materials forms a complex with the ligand on the membrane pore surface and is subsequently displaced in a concentrated state.
In any pressure driven process with a polymeric material or suspension, a process known as concentration polarization occurs which results in substantial loss of permeation due to fouling of the membrane surface. This phenomenon limits the concentration of polymer or suspended absorbent which can be efficiently used.
U.S. Pat. No. 4,474,690 is directed toward a method for recovering peptide-containing compounds from solution and employs a carrier and a ligand selected for biospecific affinity for the peptide-containing compounds. The method employs a semipermeable membrane and pressure to drive the peptide-containing compound to the carrier/ligand complex across the membrane after which the peptide-containing compound is stripped from the complex.
Despite the widespread existence of different processes for the selective extraction of various materials, the art has not recognized heretofore a scrubbing process wherein feed stream solutions containing mixtures of dissolved metal ions or other different materials can be treated in high concentrations.